Standardize capitalization of RuntimeException

Fixes #23
Check Jenkins response in getJob() and return false for 404s

Instead of throwing a `RuntimeException` about `json_decode`, return
false if the job doesn’t exist.

Fixes #26
Add anti-CSRF crumb (token) support

Adds the ability to use Jenkins’ anti-CSRF functionality by sending a
special ‘crumb’ header with a unique token to prevent CSRF.

Fixes #25
Merge branch 'add-crumbs'
Merge branch 'handle-404s-in-getjobs'
Merge branch 'fix-23'